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In salmon plasma/serum, three major insulin-like growth factor binding proteins (IGFBPs) are consistently detected at 22-, 28- and 41-kDa. The 22-kDa form has been identified as IGFBP-1b and shown to increase under catabolic conditions. We developed a competitive time-resolved fluoroimmunoassay (TR-FIA) for salmon IGFBP-1b. Purified salmon IGFBP-1b was used for biotin-labeling, assay standard and antiserum production. The TR-FIA did not cross-react with the 41-kDa form (IGFBP-2b) but showed 3% cross-reactivity with the 28-kDa form (IGFBP-1a). It measured IGFBP-1b levels as low as 0.4 ng/ml, and ED 80 and ED 20 were 0.9 and 24.6 ng/ml, respectively. There appears to be little interference by IGF-I. Using the TR-FIA, serum IGFBP-1b levels were measured in individually-tagged underyearling masu salmon fed or fasted for 5 weeks, or fasted for 3 weeks followed by refeeding for 2 weeks. Fasting for 3 weeks significantly increased circulating IGFBP-1b levels, while it returned to the basal levels after prolonged fasting for additional 2 weeks. Serum IGFBP-1b level negatively correlated with body weight, condition factor, specific growth rate and serum IGF-I level. During parr–smolt transformation of masu salmon, average circulating IGFBP-1b levels were the highest in May. There was a positive correlation between serum IGFBP-1b and IGF-I, which is in contrast to that in the fasting/feeding experiment. IGFBP-1b also showed a positive relationship with gill Na + , K + -ATPase activity. These results suggest that the relationship between circulating IGFBP-1b and IGF-I during smoltification differs from that during fasting and IGFBP-1b may play a role in the development of hypoosmoregulatory ability. [ABSTRACT FROM AUTHOR]

Abstract: Two subtypes of insulin-like growth factor binding protein (IGFBP)-1 are present in salmon blood and they are both up-regulated under catabolic conditions such as stress. The present study examined effects of fasting and re-feeding on IGFBP-1a (28-kDa form) and IGFBP-1b (22-kDa form) both at mRNA and protein levels along with IGF-I and RNA/DNA ratio in yearling masu salmon. Fish were individually tagged and assigned to one of three treatments: Fed, Fasted or Re-fed. Circulating IGF-I levels significantly decreased after fasting for 5weeks and were positively correlated with individual growth rates. Liver igf-1 mRNA levels were not affected by the treatment. Muscle RNA/DNA ratio did not respond to fasting nor showed correlations with growth rates. Circulating IGFBP-1a and IGFBP-1b increased during fasting and decreased after re-feeding. Both serum levels were inversely correlated with growth rates, while IGFBP-1b had consistent negative relationships with growth rates. Fasting/re-feeding also affected their mRNA levels in the liver. These results suggest that circulating IGF-I and IGFBP-1b could serve as positive and negative indices of growth, respectively, in masu salmon. Different sensitivities of IGBP-1a and IGFBP-1b may be useful to assess a broad range of catabolic conditions when they are combined. [Copyright &y& Elsevier]